The European Telecommunications Standardisation Institute (ETSI) is currently in the process of standardising a new set of protocols for mobile telecommunications systems. The set of protocols is known collectively as the Universal Mobile Telecommunications System (UMTS). FIG. 1 illustrates schematically a UMTS network 1 which comprises a core network 2 and a UMTS Terrestrial Radio Access Network (UTRAN) 3. The UTRAN 3 comprises a number of Radio Network Controllers (RNCs) 4, each of which is coupled to a set of neighbouring Base Transceiver Stations (BTSs) 5. Each BTSs 5 is responsible for a given geographical cell and the controlling RNC 4 is responsible for routing user and signalling data between that BTS 5 and the core network 2. All of the RNCs are coupled to one another. A general outline of the UTRAN 3 is given in Technical Specification TS 25.401 V2.0.0 (1999-09) of the 3rd Generation Partnership Project, ETSI.
User and signalling data may be carried between an RNC and a mobile terminal (referred to in UTRAN as User Equipment (UE)) using Radio Access Bearers (RABs). Typically, a mobile terminal is allocated one or more Radio Access Bearers (RABs) each of which is capable of carrying a flow of user or signalling data. RABs are mapped onto respective logical channels. At the Media Access Control (MAC) layer, a set of logical channels is mapped in turn onto a transport channel, of which there are two types: a “common” transport channel which is shared by different mobile terminals and a “dedicated” transport channel which is allocated to a single mobile terminal. One type of common channel is a Forward Access CHannel (FACH). A basic characteristic of a FACH is that it is possible to send one or more fixed size packets per transmission time interval (10, 20, 40, or 80 ms). However, in any one given time interval all of the transmitted packets must be of the same length. Several transport channels (e.g. FACHs) are in turn mapped at the physical layer onto a Secondary Common Control Physical CHannel (S-CCPCH) for transmission over the air interface between a BTS and a mobile terminal.
When a mobile terminal registers with an RNC, via a BTS, that RNC acts at least initially as both the serving and controlling RNC for the mobile terminal. The RNC both controls the air interface radio resources and terminates the layer 3 intelligence (Radio Resource Control (RRC) protocol), routing data associated with the mobile terminal directly to and from the core network. FIG. 2 illustrates the protocol model for the FACH transport channel when the serving and controlling RNCs are coincident and where Uu indicates the interface between UTRAN and the mobile terminal (UE), and Iub indicates the interface between the RNC and a NodeB (where NodeB is a generalisation of a BTS). It will be appreciated that the MAC (MAC-c) entity in the RNC transfers MAC-c Packet Data Units (PDUs) to the peer MAC-c entity at the mobile terminal, using the services of the FACH Frame Protocol (FACH FP) entity between the RNC and the NodeB. The FACH FP entity adds header information to the MAC-c PDUs to form FACH FP PDUs which are transported to the NodeB over an AAL2 (or other transport mechanism) connection. An interworking function at the NodeB interworks the FACH frame received by the FACH FP entity into the PHY entity.
Consider now the situation which arises when a mobile terminal leaves the area covered by a RNC with which the terminal is registered, and enters the area covered by a second RNC. Under the UTRAN protocols, the RRC remains terminated at the first RNC whilst the terminal takes advantage of a cell and common transport channel of the second RNC. Thus, the first RNC remains as the serving RNC with a connection to the core network whilst the second RNC becomes the controlling RNC. The controlling RNC is in control of the NodeB where the mobile terminal is located and in particular of the logical resources (transport channels) at that NodeB. In this scenario the controlling RNTC is referred to as a “drift” RNC (the controlling RNC will also be acting as a serving RNC for mobile terminals registered with that RNC). The protocol model for the FACH transport channel when the serving and controlling RNCs are separate is illustrated in FIG. 3. It will be noted that a new interface Iur is exposed between the serving and the controlling RNCs. An Iur FACH FP is used to interwork the Common MAC (MAC-c) at the controlling RNC with the Dedicated MAC (MAC-d) at the serving RNC.
In both of the scenarios illustrated in FIGS. 2 and 3, an important task of the MAC-c entity is the scheduling of packets (MAC PDUs) for transmission over the air interface. If it were the case that all packets received by the MAC-c entity were of equal priority (and of the same size), then scheduling would be a simple matter of queuing the received packets and sending them on a first come first served basis. However, UMTS defines a framework in which different Quality of Services (QoSs) may be assigned to different RABs. Packets corresponding to a RAB which has been allocated a high QoS should be transmitted over the air interface as a high priority whilst packets corresponding to a RAB which has been allocated a low QoS should be transmitted over the air interface as a lower priority. Priorities are determined at the MAC entity (MAC-c or MAC-d) on the basis of RAB parameters.
UMTS deals with the question of priority by providing at the controlling RNC a set of queues for each FACH. The queues are associated with respective priority levels. An algorithm is defined for selecting packets from the queues in such a way that packets in the higher priority queues are (on average) dealt with more quickly than packets in the lower priority queues. The nature of this algorithm is complicated by the fact that the FACHs which are sent on the same physical channel are not independent of one another. More particularly, a set of Transport Format Combinations (TFCs) is defined for each S-CCPCH, where each TFC comprises a transmission time interval, a packet size, and a total transmission size (indicating the number of packets in the transmission) for each FACH. The algorithm must select for the FACHs a TFC which matches one of those present in the TFC set.